This invention relates to planetary gear transmissions of the split power path type having a variable speed drive unit in one of the power paths for varying the overall drive ratio of the transmission in an infinitely variable manner.
Split power path types of planetary gear transmissions with infinitely variable speed drive units are well known. Generally, such transmissions include a power path extending through the variable speed drive unit which transmits at times a substantial portion of the input torque. Accordingly, the variable speed drive unit must be designed to have a high torque transmitting capacity and must therefore be made of a relatively large size. Further, the type of variable speed unit selected must often be compatable with drive ratio change in a precise and smooth manner through a relatively wide change speed range including both reverse and forward drive ratios. While traction drive types of variable speed drive units could be ideally suited for such purposes, they are subject to rapid wear of the traction engaging surfaces and slippage under high torque conditions. The use of high contact pressures between the engaging surfaces to minimize slippage and meet the torque loading increases both mechanical losses and the rate of wear. Attempts to vary the contact pressure as a function of the centrifugal force or rotational speed of traction drive cone elements for example, have not proved to be a very practicable solution in heavy duty transmissions utilized for propelling automotive vehicles. It is therefore an important object of the present invention to provide a heavy duty transmission of the split power path type capable of providing a wide range of drive ratios under continuous control of a variable speed drive unit in one power path having a smaller drive ratio range. It is a further object to transmit a relatively low torque through the aforesaid variable drive unit under an optimum contact pressure to minimize wear and shear stresses as well as to reduce mechanical losses.